Extend that logic beyond the topic of books, and you start to see the problem with it. I can't afford to eat lobster whenever I want, why shouldn't I rob the Red Lobster. I don't have the cash to buy a Aston Martin, why shouldn't I carjack the next one that rolls by?
Plus you have an ethical alternative.
Sites like Project Gutenberg.org offers thousands of book titles that have fallen out of copyright, absolutely free.
Also, there's your public library, where you can borrow paper, and e-book copies of newer titles. This is also completely free.